A number of ultrasonic devices have heretofore been proposed for use in ablating or removing obstructive material from blood vessels. Examples of devices which purportedly utilize ultrasonic energy, alone or in conjunction with other treatment modalities, to remove obstructions from blood vessels include those described in U.S. Pat. Nos. 3,433,226 (Boyd), 3,823,717 (Pohlman, et al.), 4,808,153 (Parisi), 4,936,281 (Stasz), 3,565,062 (Kuris), 4,924,863 (Sterzer), 4,870,953 (Don Michael, et al.), 4,920,954 (Alliger, et al.), and 5,100,423 (Fearnot) as well as other patent publications W087-05739 (Cooper), W089-06515 (Bernstein, et al.), W090-0130 (Sonic Needle Corp.), EP316789 (Don Michael, et al.), DE3,821,836 (Schubert) and DE2,438,648 (Pohlman).
Ultrasound transmitting catheters have been utilized to successfully ablate various types of obstructions from blood vessels of humans and animals. Particular success has been observed in ablation of atherosclerotic plaque or thromboembolic obstructions from peripheral blood vessels such as the femoral arteries. Successful applications of ultrasonic energy to smaller blood vessels, such as the coronary arteries, necessitates the use of ultrasound transmitting catheters which are sufficiently small and flexible to permit transluminal advancement of such catheter through the tortuous vasculature of the aortic arch and coronary tree.
Additionally, ultrasound transmitting catheters may be utilized to deliver ultrasonic energy to blood vessel walls for purposes of preventing or reversing vasospasm as described in copending U.S. patent application Ser. No. 07/911,651, entitled ANGIOPLASTY AND ABLATIVE DEVICES HAVING ONBOARD ULTRASOUND COMPONENTS AND DEVICES AND METHODS FOR UTILIZING ULTRASOUND TO TREAT OR PREVENT VASOSPASM.
In utilizing ultrasound catheters which are of a significant length i.e., longer than approximately 50 cm, additional problems arise. The dissipation of heat generated by the ultrasound transmission member within a catheter is one particular concern. Although it is known to flush coolant, i.e., saline, through the catheter to aid in the dissipation of heat, such procedure is not always adequate.
Bends formed in the ultrasound catheter as it passes through various anatomical vessels of the human body exacerbate the problem of heat dissipation. Such bends provide points of frictional contact between the catheter and the ultrasound transmission member, thus resulting in excessive heat build-up.
As those skilled in the art will appreciate, the build-up of heat within the ultrasound catheter can result in damage to the catheter as well as ineffective operation thereof.
One solution for mitigating the effects of heat build-up within such ultrasound catheters in the prior art has been to pulse the ultrasonic transducer such that ultrasound is applied to the ultrasound transmission member only intermittently. Such devices typically utilize a duty cycle of approximately 30-50%. Thus, heat build-up ceases during the 70-50% off portion of the duty cycle when no ultrasound is being applied to the ultrasound transmission member, thus providing for cooling of the ultrasound catheter.
Those skilled in the art will appreciate that a one hundred per cent duty cycle is defined by an output signal which is not modulated. In applications where heat dissipation is not a problem, such a one hundred per cent duty cycle may be utilized.
However, the use of such pulsing techniques increases the probability of ultrasonic transmission member breakage due to the large acceleration gradients experienced during the abrupt on/off cycling of the ultrasound transducer. Such large acceleration gradients result in ultrasound operational frequencies in excess of 20 KHz being applied to the ultrasound transmission member. It is preferred that operation ultrasound frequencies be maintained at less than approximately 20 KHz so as to limit the large acceleration gradients associated with higher operational ultrasound frequencies.
Those skilled in the art will recognize that such large acceleration gradients are commonly associated with on/off cycling of oscillating mechanical systems and are due to the extremely short time duration over which the system changes from the rest state to the moving state and vice versa.
Additionally, such pulsed systems typically provide reduced ablation efficacy due to the lengthy, i.e., 70-50%, off portion of the duty cycle.
As such, it would be beneficial to provide an ultrasound catheter which is not subject to the problems of inadequate heat dissipation and ultrasound transmission member breakage due to on/off cycling.
Furthermore, contemporary ultrasound catheters are subject to providing ineffective ablation due to variations in the intensity of the ultrasound energy provided at the distal tip of the ultrasound transmission member. Such variations in the intensity of the ultrasound energy at the distal tip of the ultrasound transmission member reduce the efficacy of the ablation process by causing the ultrasound catheter to operate at a lower than desired level. That is, such variations in intensity cause the distal tip of the ultrasound transmission member to radiate less than the desired intensity of ultrasound energy, even though the ultrasound transmission member may be driven at or near its maximum safe level.
Such variations in the intensity of the ultrasound energy radiated at the distal end of the ultrasound transmission member are typically due to bends encountered in the tortuous vessels of the human anatomy, particularly when accessing small vessels such as the coronary artery from remote locations such as the femoral artery. During such procedures the ultrasound catheter is required to make several sharp bends. Such bends inherently result in a reduction in the intensity of the ultrasound energy transmitted through the ultrasound transmission member. This is thought to be due to radiation and reflection losses inherent in transmitting ultrasound energy through such a bent transmission member and also due to frictional engagement of the ultrasound transmission member with the catheter at the site of the bend.
As such, it would be beneficial to provide an ultrasound catheter which substantially overcomes the problem of inadequate ablation efficacy due to ineffective ultrasound energy transmission, i.e., loose mechanical connection and/or bends in the catheter's path.